1. Field of the Invention
The present invention relates to an image forming apparatus including a development device, such as a copying machine, a printer, a recorded image display apparatus, and a facsimile.
2. Description of the Related Art
Recently, in an image forming apparatus using an electrophotographic system, such as a copying machine and a printer, the apparatus body is strongly required to be reduced in size. Especially in a full-color image forming apparatus, since a plurality of development devices is arranged, the size reduction is especially strongly desired. Thus, there has been known a vertical agitation type development device in which space-saving is realized in comparison with a horizontal agitation type development device (for example, see, Japanese Patent Laid-Open No. 5-333691).
In the vertical agitation type development device, two conveying screws for agitating and mixing a two-component developer are arranged vertically. A first conveying screw conveys a developer in a developing chamber above a development container to supply the developer to a developing sleeve. A second conveying screw conveys the developer in an agitating chamber below the development container and, at the same time, mixes and agitates the developer collected from the developing sleeve and the developer newly supplied.
In the vertical agitation type development device, the developing chamber and the agitation chamber are arranged in the vertical direction as described above, so that the occupied space in the horizontal direction may be small. Thus, even in a tandem system full-color image forming apparatus in which a plurality of developing devices is arranged in parallel in a horizontal direction, for example, the size reduction can be improved.
In the horizontal agitation type development device, the developer is supplied from the developing chamber to the developing sleeve, and the developer after development is collected in the developing chamber again. Thus, the developer whose toner concentration is reduced by development may be supplied to the developing sleeve while being not fully agitated. However, in the vertical agitation type development device, the developer is only supplied from the upper developing chamber to the developing sleeve, and the developer after development is not collected. As described above, since the functions are separated for each chamber, a developer having a uniform toner concentration can be supplied to the developing sleeve.
However, on the other hand, a function separating type development device like the vertical agitation type development device has the following problem. FIG. 14 is an explanatory view for describing a developer surface height variation in the prior-art development device.
As illustrated in FIG. 14, the vertical agitation type development device includes a developing chamber 123 and an agitation chamber 124 separated by a bulkhead 127. A first conveying screw 125 of the developing chamber 123 conveys a developer in the longitudinal direction and, at the same time, supplies a part of the developer to a developing sleeve. Thus, the amount of the developer conveyed by the first conveying screw 125 becomes smaller as it is conveyed further downstream in the conveying direction.
Meanwhile, the developer coated on the developing sleeve passes through a development area and is thereafter temporarily collected by a second conveying screw 126 of the agitation chamber 124. The amount of the developer conveyed by the second conveying screw 126 is increased by the amount of the developer collected from the developing sleeve. Thus, the developer amount becomes larger as the developer is conveyed further downstream in the conveying direction. Accordingly, as illustrated in FIG. 14, in the vertical agitation type development device, the height of an upper surface of the developer (developer surface height) is not fixed in the longitudinal direction, and the developer surface is inclined.
As described above, in the vertical agitation type development device, the developer surface height on the downstream side of the developing chamber (a portion A of FIG. 14) tends to become lower. Thus, when the developer surface height varies for some reasons, the developer surface height of the portion A is further reduced, and it is considered that the amount of the developer supplied to the developing sleeve becomes insufficient. In this case, a part of the developing sleeve may be not coated thereon with the developer, or a phenomenon that coating is failed (hereinafter referred to as a coating failed phenomenon) may occur.
Switching an image forming speed upon printing a thick paper and so on is one of the factors that the developer surface height varies.
In an image forming apparatus having a low-speed mode in which when a thick paper or the like is printed, image formation is performed at a lower speed than a normal image forming speed (normal mode), the rotation speeds of the developing sleeve and the conveying screw are generally reduced corresponding to a change to the low-speed mode. In such an image forming apparatus, after printing is performed in the normal mode, when the image forming mode is switched to the low-speed mode to perform printing, the coating failed phenomenon may occur due to the developer surface height variation only immediately after switching to the low-speed mode.
When the image forming mode is switched to the low-speed mode, the respective rotary drives are performed at the rotation speed for the low-speed mode. When the developing sleeve is rotated at the rotation speed for the low-speed mode, the coating amount of the developer on the developing sleeve may be different from that in the normal mode. This is because the conveying property of the developer is changed by the rotation speed of the developing sleeve. The cause that the conveying property of the developer is changed by the rotation speed of the developing sleeve is considered as follows.
The developer on the developing sleeve is caught by concaves and convexes formed on a surface of the developing sleeve by blast processing and so on, whereby the developer is conveyed in the rotating direction. Thus, the conveying property is changed by the degree that the developer is caught by the concaves and convexes on the sleeve surface.
When the rotation speed of the developing sleeve is relatively low, the developer is firmly caught by the concaves and convexes on the sleeve surface. Accordingly, the conveying property is high. Meanwhile, when the rotation speed of the developing sleeve is high to some extent, the developer is less likely to be caught by the concaves and convexes on the sleeve surface. Consequently, a slip occurs.
As described above, in comparison with the normal mode in which the developing sleeve speed is high, in the low-speed mode in which the developing sleeve speed is low, the slip phenomenon is less likely to occur. Accordingly, the developer coating amount on the developing sleeve is increased.
The slip phenomenon more notably occurs when the developing sleeve surface is worn by the use for long periods to reduce the surface roughness. Thus, as the developing sleeve is used for longer periods, the developer coating amount is more widely different between the normal mode and the low-speed mode.
When the developer coating amount on the developing sleeve increases in the low-speed mode, the amount of the developer conveyed from the developing chamber to the agitation chamber by the developing sleeve increases more than in the normal mode. Thus, the developer amount in the developing chamber is temporarily reduced in comparison with the normal mode, and the developer surface height is reduced. Accordingly, if the image forming mode is switched from the normal mode to the low-speed mode when the developer surface height in the developing chamber is low from the beginning, the developer surface is further lowered immediately after switching to the low-speed mode by the above reasons. As a result, the coating failed phenomenon of the developing sleeve may occur at a downstream portion of the first conveying screw 125 (the portion A of FIG. 14).
As described above, the developer surface height in the developing chamber is lowered immediately after the image forming mode is switched to the low-speed mode, and meanwhile, the developer amount in the agitation chamber 124 is increased. Consequently, the amount of the developer drawn up from the agitation chamber 124 to the developing chamber 123 is increased. After a period of time, the developer amount in the developing chamber 123 is recovered, and the coating failed phenomenon does not occur. Namely, the coating failed phenomenon most likely occurs immediately after switching to the low-speed mode.